Study of Wear Resistance of Plasma Electrolytic Oxidized Coatings
on Aluminum Alloys
Y. Kuznetsov1, A. Kossenko2, A.Lugovskoy2
1
2
Orel State Agricultural University, Orel, Russia
Ariel University Center of Samaria, Ariel, Israel
ABSTRACT
Results of experimental studies of oxide-ceramic coatings obtained by Plasma
Electrolytic Oxidation (PEO) on aluminum alloys in two electrolytes are reported.
Introduction
One of the most actual tasks of the modern technology is the development of
environmental friendly methods to produce highly effective and reliable coatings
improving the wear resistance of different machine details. Plasma Electrolytic
Oxidation (PEO) is one of the most promising techniques of production of modified
ceramic coatings having a wide spectrum of favorable properties. The idea of the
technique lies in the formation on the aluminum surface of hard and wear-resistant
oxide ceramic coating consisting of -Al2O3 and other aluminum oxides, under the
impact of micro-arc discharges.
On one hand, there are considerable gaps in the theoretical understanding of
PEO process: various aspects of the mechanism are still unclear, the comprehensive
knowledge of the influence of various factors on the PEO process are lacking, many
potentially perspective electrolytes have not been tested. On the other hand, the
practice demands the investigation of tribological characteristics of the coatings,
whose most complete understanding can be achieved by wear tests.
Experimental
The following aluminum alloys were studied: cast alloy AK7ch GOST* 1583,
antifriction alloy AO3-7 GOST 14113, industrial wrought alloys AD1 (1013), Amg2
(1520), D16 (1160) GOST 4784. The choice of alloys was determined by their wide
usage in agricultural and tractor machinery. The chemical composition of the alloys is
given in Table 1.
Combined potassium hydroxide based electrolytes were chosen for the PEO
processes:
(1) an electrolyte with "liquid glass" additive ("KOH – Na2SiO3 system")
(2) an electrolyte with boric acid additive ("KOH – H3BO3 system"). Starch was
also added to this type of electrolytes for the enhancement of its stability and
efficiency [1].
The working solutions were prepared by dissolving of analytical grade reagents in
distilled water. Weighing was performed on VLKT-500g-M laboratory balance.
*GOST is the designation of technical standards maintained by the Euro-Asian Council for
Standardization, Metrology and Certification (EASC), a regional standards organization operating
under the auspices of the Commonwealth of Independent States (CIS).
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Potassium hydroxide GOST 24363 meas.1, liquid glass GOST 130078 silicate index
m = 3.0 and density ρ = 1.47∙103 kg / m3, boric acid TU 6-09-17-263 and starch
GOST 7699 were used.Coating in "KOH – Na2SiO3 system" was performed at the
current density of 20 A / dm2, time of 120 minutes, KOH concentration of 2 g / L and
Na2SiO3 concentration of 10 g / L. In "KOH – H3BO3 system") the coating was
performed at the current density of 20 A / dm2, time of 100 minutes, KOH
concentration of 5 g / L, boric acid concentration of 20 g / L and starch concentration
of 10 g / L.
Weight %
Alloy
Mg
Si
Mn
Cu
Ti
Ni
-
-
Сast alloys GOST 1583
Al - Si – Mg system
AK7ch (Al9)
0.2-0.4
6.0-8.0
-
-
Wrought alloys GOST 4784
Fe 0.3
Zn 0.1
Fe 0.5
Cr 0.10
Zn 0.25
Fe 0.5
Cr 0.05
Zn 0.15
AD1 (1013)
0.05
0.3
0.025
0.5
0.15
D16 (1160)
1.2-1.8
0.5
0.3-0.9
3.84.5
0.15
Amg2 (1520)
1.7-2.4
0.4
0.1-0.5
0.15
0.15
АО3-7 (14113)
Sn
Mg
Mn
Sb
Cu
Ni
Si
2.5-3.5
-
0.5-0.8
-
7.0-8.0
-
0.6-1.2
Table 1. Alloys composition in mass %. GOSTs are designated in parentheses.
The oxidation was performed on a home-made PEO station with 40 kVA
output.
Fig. 1. Samples for the microhardness measurements.
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Microhardness was measured on PMT-3M device according to GOST 9450 on
cross sectioned samples both along the section line and perpendicularly to the coating
layer. The load on the indenter was varied in the range 0.981 – 1.962 N. The coated
samples (Fig. 1) were cut and treated according to the recommendations [2]. Prior to
PEO disks were polished on round-polishing machine 3M150 so that no visible
grinding splits or surface damage appeared and the surface finish remained in the
range of Ra = 0.25 – 0.40 μm. After the oxidation samples were ground on abrasive
paper to Ra = 0.20 – 0.25 μm.
Prior to the microhardness measurements perpendicular to the surface coating
layer the samples were ground to the removal of the loose layer and then polished.
Microhardness Hμ was determined according to tables [3].
(а)
(b)
Fig. 2. Samples for wear resistance tests on II 5018 friction machine: (a) disk,
(b) counter-sample.
Fig. 3. Wear test scheme. 1: counter-sample, 2: dosing valve, 3: electric motor, 4:
stirrer, 5: oil reservoir, 6: sample disk.
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Wear resistance of PEO hardened AK7ch and AO3-7 samples (Fig. 2) was
measured on friction machine II 5018 under boundary lubrication conditions
according to the scheme shown in Fig. 3.
The counter-sample (Fig. 3-1) was made of 18HGT Steel having the hardness
HRC 58-62, contact area 2 cm2 and surface finish Ra = 0.16 μm. The choice of
materials was determined by the fact that 18HGT Steel is used for the production of
gears, AK7ch alloy for the body and AO3-7 alloy for the bushings of NSH and NSHU pumps. The above pump parts have the same technological parameters as those
used in the preparation of samples.
Uncoated samples of AK7ch and AO3-7 alloys having the surface finish of Ra = 0.20
– 0.25 μm were taken as reference. The friction velocity was 0.52 m / s. Boundary
lubrication conditions were maintained by the steady supply of spindle oil AU GOST
1642 to the friction surface. Abrasive mixture made of quartz sand having the
dispersity of 3μm was added to the working fluid for the wear enhancement. The
concentration of the abrasive additive in the oil was 0.14 mass % [4,5].
The counter-sample was held in a floating cage for the maximal adjoining of
friction surfaces and equal distribution of the load. The formation of the working
relief occurred while the load was continuously changed from 0.25 to 0.95 MPa. After
achievement of constant friction moment and at least 90% breaking in of the friction
surfaces, the load was increased to that of the experiment (3.0 MPa) by 0.25 MPa
steps.
The values of wear were determined by weighing of the sample and countersample (on ADV-200M balance). The test time was 50 hours.
Fig. 4. Experimental friction machine reproducing the kinematic work type of "saddle –
ball-cock" junction of the Zh6-VNP pump valve-box designed for the flow of liquid
food products
Experimental friction machine (Fig. 4) was designed and manufactured for the
wear tests of the hardened layers on wrought aluminum alloys AMg2, AD1 and AD6
in "KOH-H3BO3" electrolytes. The machine reproduces the kinematic work type of
"saddle – ball-cock" junction of the Zh6-VNP pump valve-box designed for the flow
of liquid food products. The kinematic type of the experimental machine was
equivalent to the above pump by the position, geometry and character of relative
movements of the friction surfaces. The experimental machine maintained the
working load tripled as compared to the pump and 1 hour of its working time was
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equivalent to 5 hours for the pump. The machine achieved the working resource of the
pump in 72 hours by the producing of 3 million load cycles on the junction.
The reference for the hardened samples was BrA5 bronze, of which valveboxes of the above pumps are produced. The working media was imitated by 3%
lactic acid solutions.
The applied objective of the research was the restoration of worn valve-boxes using
PEO hardened aluminum alloy bushings.
The evaluation of wear resistance of hardened samples (of wrought aluminum
alloys) was made by the calculation of mean wear intensity I determined by equation
(1):
I
Wl
L fr
,
(1)
where Wl is the linear wear of a sample, m; Lfr is the corresponding friction path, m.
The linear wear Wl was determined by equation (2):
Wl 
G
,
  Acont
(2)
where ΔG is the change of mass in the course of the test, kg; γ is the density of worn
material,
kg / m3; Acont is the contour area of the friction contact, m2.
The value of Lfr was determined as:
L fr  N  l fs ,
(3)
where lfs is the linear dimension of the friction surface for a conjugated sample along
the slide direction, m; and N is the number of cycles of passing the path of lfr by the
driction surfaces.
Results and discussion
1. Wear resistance of PEO coatings on aluminum alloys in "KOH – Na2SiO3 system"
electrolytes
The wear tests of the friction couples demonstrated (Fig. 5) that the
dependence of wear on time was linear. The wear rates of uncoated AK7ch and AO37 alloys, which had been accepted as references, were determined as 0.036 g / hr and
0.019 g / hr correspondingly. The wear rates of these alloys with PEO coatings were
0.0084 g / hr for AK7ch and 0.004 g / hr for AO3-7. That is, PEO coating reduces the
wear rates by the factor of 4.3 for AK7ch alloy (Fig. 5a) and by the factor of 4.8 for
AO3-7 (Fig. 5b).
The best wear resistance observed for AO3-7 alloy can be explained by the
anti-friction effect of copper in its composition.
Some increase in the wear rate of steel anti-bodies was observed for their friction by
coated samples (by 16.8% for AK7ch and 12% for AO3-7 alloys, Fig. 5). However,
the integral wear resistance of friction couples for the samples having ceramic PEO
coatings was higher than that for the uncoated samples by the factor of 1.6 (AK7ch)
and 1.8 (AO3-7).
Working surfaces of the reference (uncoated samples) friction couples after the
friction tests were covered with numerous lateral grooves and scratches appeared as
the result of the abrasive particles impact on the metal (Fig. 6). Contrasting to that,
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grooves and scratches were apparently not observed on the working surfaces of
friction couples with PEO coated samples. This is a direct evidence of the higher wear
resistance of friction couples with PEO coated aluminum alloys.
(а)
(b)
Fig. 5. Time dependencies of wear: (a) 1: uncoated AK7ch sample; 2: PEO coated
AK7ch sample; 1* and 2*: corresponding 18HGT steel (HRC 58 – 62) countersamples. (b) 1: uncoated AO3-7 sample; 2: PEO coated AO3-7 sample; 1* and 2*:
corresponding 18HGT steel (HRC 58 – 62) counter-samples
Sample
Counter-sample
Uncoated AO3-7 alloy
18HGT Steel
PEO coated AO3-7 alloy
18HGT Steel
Fig. 6. Friction surfaces after wear tests performed on II 5018 friction machine
(magnification x24)
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The reasons to the higher wear resistance of the PEO coated alloys are
obviously connected with the coating structure. It can be assumed that the structural
modification of aluminum oxide in the coatings for the strongest intermolecular bonds
[6]. Some authors note that PEO oxide coatings are, in fact, composite materials [7-9].
Some additional explanation to the higher wear resistance of PEO coatings
formed in "KOH – Na2SiO3 system" electrolytes could be found in their higher
microhardness. It is worth to be noted that the microhardness is unevenly distributed
over the coating thickness (Fig. 7). The maximal microhardness is observed in the
layer found at 20 – 40 μm outside the nominal sample border, while any other layer
has lower microhardness.
Fig. 7. Microhardness profile (current density 10 A / dm2, oxidation temperature
40°C, oxidation time 120 min, 2 g / L KOH, 10 g / L Na2SiO3). 1: AK7ch alloy, 2:
AO3-7 alloy.
The higher microhardness and its uneven distribution of PEO coatings on aluminum
alloys have also been reported by other authors [10, 11, 12]. It has been also reported
that due to higher coating discharge temperature the fraction of α- and γ-Al2O3 is
higher in inner layers than in outer layers of the coatings [9]. When practically
reasonable oxidation regimes are used, the mean microhardness of the PEO coatings
on AK7ch and AO3-7 alloys is in the range of 7 – 8 Gpa (cf. 0.94 – 0.99 Gpa for the
uncoated alloys).
It has been noted [11, 12] that the wear resistance of PEO coatings is
comparable to that of composite materials based on tungsten carbide, which are
traditionally used to resist abrasion wear.
Therefore, it can be summarized that the PEO coatings obtained in the "KOH Na2SiO3 system" electrolytes ensure higher wear resistance and can be recommended
for the surface hardening of various machine parts and devices.
1. Wear resistance of PEO coatings on aluminum alloys in "KOH – H3BO3 system"
electrolytes
The tests showed (Fig. 8) that the wear intensity of reference bronze surface
was 2-3 times higher than that of wrought aluminum alloys hardened by PEO in
"KOH – H3BO3 system" electrolytes. The lower, as compared to the reference, wear
intensity of oxide-ceramic coated aluminum alloys can be explained by the higher
microhardness of the surface layers (Fig. 9). The hardening phases in the coatings
obtained in "KOH – H3BO3 system" electrolytes are α- and γ-Al2O3, the high harness
of the former (9 on Mohs scale [13]) allows its industrial use as an abrasive material.
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The coatings on Amg2 and AD1 alloys have higher microhardness and wear
resistance than those on D16 alloy. This might assume that the coatings on the latter
alloy have lower fracture of the hardening phases.
Fig. 8. Wear intensity (J) of oxide-ceramic coated aluminum alloys as compared to
BrAl5 bronze.
Fig. 9. Surface microhardness of PEO coatings on wrought aluminum alloys formed
under various current densities for Amg2 (left), AD1 (middle) and D16 (right) alloys.
PEO parameters were t = 120 min, T = 40°C, CKOH = 5 g /L, CH3BO3 = 15 – 30 g /L.
An additional advantage of the coated aluminum alloys is the fact that while
the coated surface layer has high microhardness, the aluminum substrate layer is very
plastic and this improves the alloy resistance to impact loads. The experiments
demonstrated that the coated samples showed no deterioration even after 3 million
load cycles.
Another destructive factor impacting the samples is the medium, which
contained an organic acid solution. The visual and microscopic observation
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demonstrated that the reference surface after the tests was pierced by cracks (to 0.5
mm), which were aligned along the friction surface. These cracks are apparently
formed because of surface tension strain causing corrosion cracking process. The
working medium inside a crack creates some excess pressure, which favors the further
development of the crack. No such cracks were detected in PEO coated samples and it
supports the observation that compression rather than tension strains are formed in
oxide-ceramic coatings [14, 15].
Conclusions
The complex tests of oxide-ceramic PEO coatings on aluminum alloys demonstrated
that these coatings have significantly higher wear resistance and microhardness as
compared to reference aluminum and bronze samples. In some cases industrially
produced aluminum or bronze parts can be replaced by the PEO coated aluminum
alloys and their working resource cam be enhanced by 150 – 200%
References
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